It is essential to provide for the control of parameters that are significant for the integrity and proper operation of the members and components of the engine. For this, the usual method consists in taking measurements directly on these components. In the event of an abnormal variation of a factor such as temperature, pressure or stress, it is then possible to stop the system before an incident occurs and the engine ceases to operate. This is for example the case when a part is insufficiently lubricated or subjected to an excessively high temperature. According to the use that is made of the equipment, the sanitary or economic consequences can be very substantial. Allowing for an anticipated and automatic stopping of the system if an abnormal parameter is observed is a major advantage. The effectiveness of such a device is therefore to a large extent dependent on its rapidity.
Prior art describes several systems that make it possible to carry out such a control.
For example, patent document EP 0.051.035 (CMR) discloses a device for controlling the temperature of the internal mobile parts of an engine by the intermediary of a transmitter-receiver formed of two fixed coils. This transmitter-receiver emits a signal carried by a magnetic circuit resonating at a determined frequency. This signal is detected by a sensor (formed by one mobile coil) placed on a moving part, and connected to a thermistor that reacts to the temperature of the part by influencing the sensitivity of said sensor, which therefore detects more or less substantially the signal that is sent to it by the transmitter-receiver. The resonating signal is measured by the transmitter-receiver and is interpreted by an electronic device as a temperature unit.
In this device, several restricting aspects are to be noted. First of all, it is the thermistor that defines the response of the sensor according to non-linear variations, which is inconvenient since according to the quality and the precision of this element, the measurements can be distorted over wide ranges of variation. In addition, the transmitter-receiver generates a magnetic field through which the waves travel. However, this magnetic field is not entirely detected by the sensor, which results in the loss of a portion of the waves emitted and received by the transmitter/receiver. The precision of the measurement therefore depends on the quality of the coil of the system and on the meticulous calibrations to be carried out for each sensor in the factory. Then, the maximum reading distance that separates the transmitter/receiver from the sensor is only of a magnitude of 2 mm, which is rather low and can cause a problem for certain moving parts, for example in relation to the operating clearances that can be of the same order of magnitude. These problems do not make it possible to respond satisfactorily to user expectations.
Patent document WO 00/62029 (SENSIT) also discloses a system based on the use of filters of surface acoustic waves (or SAW) which consists in emitting electromagnetic waves via a stationary antenna, which are converted into mechanical waves in a device constituted of piezoelectric crystal or ceramic. This wave is diluted as it propagates through the component before it is reconverted into an electrical signal by electrodes. This response signal is reconfigured to produce a finite analog impulse response through a filter. This method is based on the principle of delay lines. The delay of the reflected wave depends on the temperature. In this system which is shown in FIG. 1, a single electronic control unit 3 calls up, via antennas 2, several SAW sensors 1 placed in each engine compartment, on the parts for which the temperature must be measured. The information is communicated by the electronic control unit 3 to monitoring equipment 6 via a digital bus 5.
This system has several disadvantages. Firstly, this is a system that calls up each SAW sensor 1 via a single and same electronic control unit 3. For this, it uses the multiplexing of antennas 2, a method making it possible to call up each sensor 1 sequentially. However, this method is restrictive since it substantially increases the waiting time between each operation of calling up sensors 1. Indeed, in this system, the single control unit 3 successively calls up the sensors 1 by first sending a signal via coaxial cables 4 connected to the antennas 2. The latter communicate with the SAW sensors 1. The response signal follows the reverse path. The reception and the processing of the multiple signals received by the electronic control unit 3 increase the duration of the analysis. In addition, the metal environment of the engine has for effect to result in the loss of a portion of these signals and therefore to reduce the effectiveness of the measurements taken. Furthermore, the process for calling up SAW sensors 1 requires that each one have an antenna 2 which is dedicated to it in the engine compartment. Each communication between the antenna 2 and its SAW sensor 1 is carried out by the intermediary of a single and same frequency for all of the antennas 2. Using this single frequency therefore imposes sequential calling up of each sensor 1 in order to prevent the interferences between the responses of nearby sensors.
One of the advantages of this system in relation to other known systems of prior art, is the greater reading distance of the SAW sensors 1 by the antennas 2, said distance is increased up to 5 cm. But this distance still remains insufficient to relieve the user from the installation constraints inside the engine. Moreover, the system requires that the control unit 3 be connected to the antennas 2 by as many coaxial cables 4 and therefore imposes space constraints and very awkward cabling outside the engine.
Patent documents EP 2.422.900 (SMS CONCAST) and U.S. Pat. No. 6,239,723 (SIEMENS) disclose systems that make it possible to call up SAW sensors, arranged respectively in a mold and in an electrical cabinet. These SAW sensors are arranged on parts that are fixed, non-mobile and easy to access, and in an environment that is much less restrictive than that of an engine.